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Thu, December 22nd, 2016 - 7:00AM
Fri, December 23rd, 2016 - 7:00AM
Heather Thamm
Avalanche risk The Bottom Line

Today the avalanche danger is  MODERATE  in the Alpine where triggering an isolated wind slab or a slab 1-2′ deep is possible on steep wind-loaded terrain features. Evaluate snow and terrain carefully and manage your terrain choices with safe travel protocols. At Treeline and below the avalanche danger is LOW.  Remember to avoid terrain traps where an avalanche of any size could have high consequences.

If you are headed to Summit Lake check out the Saturday Summary and recent Summit observations.  

Special Announcements

***Carter Lake and Snug Harbor areas are now open to motorized use as of Tuesday, December 20th. Please respect other closures across the Forest. The Forest Service is monitoring conditions daily and will open more areas just as soon as there is enough snow to prevent resource damage to underlying vegetation.  If you are headed to Snug Harbor and Carter today and please send in an  observation  if you see any obvious signs of instability.

Thu, December 22nd, 2016
Above 2,500'
2 - Moderate
Avalanche risk
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Dangerous avalanche conditions. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Wind Slabs
    Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
More info at Avalanche.org

Northwest winds15-20mph are expected in the upper elevations today. With up to a foot of snow available for transport this could easily form shallow wind slabs around leeward features in the upper elevations. A Northwest flow can sometimes cause varying wind directions for Turnagain Pass and its common to see a more Southwest direction in some places. Watch out for hard over soft snow, wind pillows or drifts and be on the look out for obvious signs like cracking and whumpfing sounds. Its also possible that a windslab could step down into an older weak layer within the snowpack. More on this below.

There is plenty of snow available for transport on all aspects. Pay attention as surface conditions change with building winds. Depending on the wind direction this Northern aspect of Magnum could see cross loading today. 

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
More info at Avalanche.org

The snowpack right now has a variety of weak layers within the top 3’ of the snowpack. A widespread layer of weak faceted snow up to 2′ deep is our primary layer of concern and was the culprit in many skier triggered avalanches over the weekend. Two seperate layers of buried surface hoar also exist within the top meter of the snowpack in some places in the alpine, but they haven’t been found everywhere. With that said these layers have presented propagation potential in some test pits this week. Should you see active wind loading today, your extra weight could be enough to tip the balance on a steep unsupported slope. Pay attention for stiff snow over weaker snow and avoid wind loaded slopes where the potential of triggering a dangerous slab avalanche is still possible with our current snowpack set-up. 

Should you go into avalanche terrain be aware of other groups and practice safe travel rituals. Only expose one person at a time, always have an escape zone, and regroup in safe spots. 


Digging a quick hand pit and poking your ski pole into the snow are good ways to evaluate the snow as you move. Be cautious around steep slopes if you find hollow snow or stiffer snow ontop of loose snow below. This photo was taken yesterday of very weak faceted snow at 2000′.  


Additional Concern
  • Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

Expect cornices to be sensitive and easy to break off. They also could trigger a slab avalanche below. If you choose to walk a ridgeline today, give these a wide berth and be aware of people below you.

 “Sluff” may also be fast moving on steep protected terrain features where the snow is loose and unconsolidated.  Be aware of terrain features that could have high consequences if knocked off your feet.

Thu, December 22nd, 2016

Yesterday a temperature inversion was the cause of valley fog and cooler air near sea level (single digits F) and warmer air in alpine (low 20F’s.) Skies were partly cloudy, winds were calm and no new precipitation was recorded.

Today Northwest winds are expected to increase this morning to a sustained15-20mph. Temperatures will range from 15-25F today. Skies should be partly cloudy and no precipitation is expected.

Northwest winds should become light early tomorrow morning with a possibility of flurries in the morning. Temperatures are expected to be in the low teens (F) in the alpine and single digits near sea level.

*** Alyeska Midway weather station only had data available from 6am to 10pm on Dec.21st, and Sunburst weather station only had data available from 12pm-7pm on Dec.21st.  

PRECIPITATION 24-hour data (6am – 6am)

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880′) 21   0   0   27  
Summit Lake (1400′) 13   0   0   9  
Alyeska Mid (1700′) ***23   ***0   ***0   ***16  

RIDGETOP 24-hour data (6am – 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812′) ***20 ***ENE ***6 ***12
Seattle Ridge (2400′) 13   ESE 1   4  
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This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area. This advisory does not apply to highways, railroads or operating ski areas.